Light microscopic localization of immunoreactive prolactin (IMP) in neuron-like cells in a number of brain areas has raised questions about the possibility of extrapituitary sites of synthesis and the routes of transport this hormone may take between CSF and/or blood and the immunoreactive cells of these loci. The proposed research is designed to explore these two questions. The soluble peroxidase-antiperoxidase (PAP) method will be used for ultrastructural localization of prolactin in cells already shown to contain IMP. Those areas of the brain to be scrutinized include the subfornical organ, pars tuberalis, area postrema, organum vasculosum of the lamina terminalis and medial habenular nucleus. Transmission electron microscopy will reveal whether these cells possess the synthetic apparatus for elaboration of protein and/or secretory granules containing immunoreactive prolactin. Autoradiographic localization of 125I-prolactin in the brain at various times after injection into either the heart or lateral cerebral ventricle is expected to provide evidence as to the route of transport of prolactin from each of these injection loci. Prolactin has been shown to influence a wide range of vertebrate biological and behavioral activities; of primary importance are those related to reproductive processes in both male and female animals. The regulation of prolactin secretion by hypothalamic amines is poorly understood and further complicated by the numerous highly variable biological and environmental factors which influence them. These studies will provide basic information concerning the location, synthesis and movement of prolactin in the brain so that the complex interrelationships between prolactin and other brain hormones may be better understood. The study will also provide morphological data for use in future studies of regulatory mechanisms for the elaboration of prolactin as well as demonstrating possible sites of action in the brain.